The present invention relates to devices for testing for fluid flow separation. In aerodynamics it is understood that the lift acting on a wing of airfoil is due to a difference in pressure between the upper and lower surfaces. This difference in pressure is maintained only if the flow follows the surface. If the fluid flow separates from the surface before reaching the trailing edge, the wing may enter a state of "stall." Separation may be defined as the phenomenon in which the boundary layer of the flow over a body in a moving stream of fluid separates from the surface of the body allowing a condition of low energy turbulent fluid to exist in the region between the body and the smooth flow. The phenomenon of flow separation depends largely upon viscous effects which are usually neglected in the circulation theory of lift. Accordingly in the study of wings, airfoils, and flow surfaces, as well as in certain practical applications, it is important and useful to have an arrangement or device for testing when fluid flow separation occurs, and where it occurs.
One example of a fluid flow separation indicator is described in U.S. Pat. No. 2,431,241 to F. W. Godsey, Jr., on Nov. 18, 1947, for "Stall Warning Indicating Apparatus." One of the defects of such a device as described in the Godsey patent which employs a housing is that the housing in itself disturbs the boundary layer of the flow over a body carrying the housing in the moving stream of fluid, and may itself induce or enhance the chances of separation of this boundary layer from the surface of the body. Some prior devices employ pressure sensing plates to detect pressures and pressure changes. Some employ conduits or passageways connecting orifices at or in the surface under test and then test for fluid flow in the passageways between the orifices, thereby testing for pressure differential between the surface portions at the orifices. These devices also introduce disturbances at the boundary layer and are sometimes not reliable in the resultant indications. Examples of fluid flow detectors, meters, or the like are described in U.S. Pat. Nos. 2,431,241 issued Nov. 18, 1947 to Godsey, Jr. for Warning Indicating Apparatus (noted above), 2,951,659 issued Sept. 6, 1960 to Yoler for Attitude Sensing and Control System, 3,196,679 issued July 27, 1965 to Howland for Fluid No-Flow Detection Apparatus, 3,366,942 issued Jan. 30, 1968 to Deane for Flow Stoppage Detector, 3,400,582 issued Sept. 10, 1968 to Warner for Boat Speed Indicator, 3,677,085 issued July 18, 1972 to Hayakawa for Tandem-Type Hot-Wire Velocity Meter Probe, 3,900,819 issued Aug. 19, 1975 to Djorup for Thermal Directional Fluid Flow Transducer, 3,945,252 issued Mar. 23, 1976 to Fiore for Flow Measuring Apparatus, and 3,922,912 issued Dec. 2, 1975 to Bradbury for Anemometers.